This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-067169, filed Mar. 9, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates generally to a radio-frequency apparatus for use in a CS (Communication Satellite) digital broadcast receiving apparatus, etc., and more particularly to an improvement of a shield mechanism for the radio-frequency apparatus.
2. Description of the Related Art
As is well known, the radio-frequency apparatus is constructed such that peripheral edge portions of a printed circuit board, on which a radio-frequency circuit is formed, is surrounded by a frame-shaped shield case. Both openings of the shield case are closed by shield covers.
In this case, each shield cover is coupled to the shield case by engaging hook-shaped resilient engaging portions formed at peripheral edges of the shield cover with projection portions formed on an outer peripheral surface of the shield case.
The printed circuit board is partitioned by a shield board into areas associated with respective circuit blocks provided on the printed circuit board. The shield board is supported on the shield case.
Each shield cover is provided with a resilient contact portion. When the shield cover is coupled to the shield case, the resilient contact portion is put in pressure-contact with the shield case or shield board, thus effecting grounding.
With this shield structure, however, a gap may be created between a terminal edge portion of the shield case and a flat surface portion of the associated shield cover, owing to dimensional errors of the shield case and shield covers.
As a result, contact between the shield case and shield covers becomes loose. Due to vibration, etc., the terminal edge portions of the shield case and the surface portions of shield covers may repeatedly come into and out of contact with each other.
The presence of such loose contact portions leads to an instantaneous change of a ground path due to vibration, noise, etc. Consequently, the oscillation frequency, etc. of, for example, a local oscillation circuit formed on the printed circuit board may vary. This variation may cause block noise in the case of digital broadcast reception.
In order to solve this problem, techniques, as disclosed in Japanese Patent No. 2,861,873, have been developed. In such techniques, however, the number of portions, at which the shield case and shield covers are opposed with a small distance, is reduced to decrease the number of locations of loose contact. These techniques provide no fundamental solution to the problem.
The present invention has been made in consideration of the above circumstances, and the object of the invention is to provide a radio-frequency apparatus capable of preventing loose contact between shield members, thus preventing a radio-frequency circuit from being adversely affected.
According to one aspect of the present invention, there is provided a radio-frequency apparatus comprising: a shield case in which a printed circuit board provided with a radio-frequency circuit is attached; a shield cover attached to the shield case such that the shield cover covers a predetermined area of the printed circuit board; and a holding mechanism configured to hold the shield cover on the shield case in a state that a predetermined distance is kept between the shield cover and the shield case, with no contact between the shield cover and terminal edges of the shield case.
Therefore, loose contact between the shield cover and the shield case can be prevented and an adverse effect on the radio-frequency circuit can also be prevented wherein the shield cover is attached to the shield case in a state that a predetermined distance is kept between the shield cover and the shield case, with no contact between the shield cover and terminal edges of the shield case.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.